In a typical hard disk drive (HDD), servo sectors on the disk are used to provide position information about the location of a magnetic head over a disk surface. A common approach for writing such servo information on one or more disk surfaces in an HDD is referred to as spiral-based self-servo writing, or spiral-based SSW. According to this approach, multiple spiral-shaped servo information patterns (or “servo spirals”) are written on at least one disk surface prior to the SSW process. During the SSW process, a magnetic head of the HDD is positioned relative to a disk surface based on timing and position information in the servo spirals, so that the final servo information (the servo sectors) can be written on the disk surface by the magnetic head.
For an error-free and robust SSW process, the servo spirals used should be precisely written on the disk surface with a predetermined and constant slope. Such servo spirals may be written on the disk surface with an external media writer before assembly of the disk drive, or with a servo writing machine that uses an external precision actuator to position the disk drive actuator with a mechanical push pin through an opening in the disk drive housing. In either case, setup and use of such external equipment for each individual HDD is time-consuming and expensive in the context of high-volume manufacturing.
In light of this, in-drive spiral-writing schemes have been employed, in which an HDD itself writes servo spirals prior to performing the SSW process. For example, as discussed in U.S. patent application Ser. No. 14/675,458, entitled “Writing Spirals with Accurate Slope on a Disk Drive Media,” filed on Mar. 21, 2015, the entire contents of which are incorporated by reference herein, a set of coarsely positioned spirals may be written by the HDD while the actuator is moved across a disk surface by applying a suitable open-loop voltage profile, or by using velocity control that is based on back electromotive force (back-EMF) feedback. More precisely positioned sets of spirals can then be written in one or more subsequent closed-loop spiral-writing processes, by demodulating signals from the more coarsely positioned sets of spirals. For example, a set of coarse servos spirals, a set of fine servo spirals, and a set of final servos spirals may be progressively written by the HDD itself in this way. The servo sectors are then written on a disk surface by positioning the magnetic head based on the final, most accurately positioned set of servo spirals.